C6 + C7 Flashcards
Define exothermic reactions
Thermal energy is transferred to the surrounds so the temp of surrounding increases
Energy from chemical energy store of system to the surrounds , so energy in system falls (negative)
Combustion, oxidation, neutralisation
Define endothermic
Thermal energy is taken from the environment and the surrounding temp lowers
Energy is transferred to the system from the surroundings so the system gains energy (positive)
Less common
Electrolysis, thermal decomposition, first stages of photosynthesis
Is bond forming exo or endothermic? (+ energy change explanation)
Exothermic
Heat released by reactants
Chemical energy is converted into thermal energy -> temp rise
Energy released is greater, so energy change is negative because products have less energy (negative ΔH value)
Is bond breaking a exo or endothermic process?
Endothermic
Thermal energy is taken-> chemical energy
More energy is needed to break the bond so the change is positive as the products have more energy than at the start (positive ΔH value)
What is activation energy?
The amount of energy needed to start a reaction, breaking bonds
Higher activation energies require more energy to start than those with lower levels
What is enthalpy change?
The transfer of thermal energy during a reaction
ΔH, of the reaction-> either positive or negative depending on if its exo or endothermic
What is bond energy?
The amount of energy required to break the bond or the amount given out when the bond is forming
Used to calculate the amount of heat taken or produced
Measured in Kj/mole
How can you determine exo or endothermic reaction from bond energy?
Balanced equation
Add together bond energy for reactants
Add together bond energy for products
Enthalpy change (energy in - energy out)
Negative-exothermic
Positive-endothermic
How do energy level diagrams look for exothermic reactions?
Reactant higher than product (goes down -> negative ΔH)
Initial increase -> Ea (activation energy) minium energy that colliding particles must have in order to react
How do energy level diagrams look for endothermic reactions?
Reactant lower than product (goes down ->positive ΔH)
Initial increase -> Ea (activation energy) minium energy that colliding particles must have in order to react
How can you investigate the rate of reactions which produce gas?
Look on the doc i cant be bothered
Rate of reaction = volume of gas produced/ time
Fastest at start
Reaction slows as time passes
Fill in the blank:
Steeper curve -> ____ rate
As reaction progresses the concentration of the reactant _____ so the rate ______ (curve less steep)
Once the reaction is over the curve is ____
Amount of product formed determined by _____ reactant
Stepper curve -> faster rate
As reaction progress the concentration of reactant decrease so the rate decreases (curve less steep)
Once the reaction is over the curve is flat
Amount of product formed determined by limiting reactant (proportional variation)
How does surface area affect the rate of reaction (experiment example)?
Look at diagram on doc (dilute hydrochloric acid)
Result:
By using different shapes of calcium carbonate we can see that the smaller sizes with bigger surface area have an increased rate of reaction
More frequent and successful collisions
How does concentration affect the rate of reaction (experiment example)?
Look at diagram on doc (dilute acid and sodium thiosulfate solution)
Results:
Increased concentration increases rate of reaction
More reactant particles allows more frequent and successful collisions
The affect of temperature on rate of reaction (experiment example)
Look on doc for diagram (dilute hydrochloric acid)
Result:
Increase temp, increase rate of reaction
The reactant particles have more kinetic energy (than required Ea) and have more frequent and successful collisions
How does the presence of a catalyst affect the rate of reaction (experiment) ?
Look on doc for diagram (hydrogen peroxide solution, manganese (IV) oxide)
Result:
Using catalyst increases rate of reaction
Catalyst provide an alternative pathway required less Ea -> more colliding particles have enough energy to react
Does not become affect -> mass doesn’t change
What is the equation for rate of reaction?
Rate of reaction= amount of reactant used or product created/time
How do temp, surface area and concentration create danger of explosive combustion in fine powders and gas?
Ex: flour mill
Surface area of powder is large
Easily combust causing explosion (flammable substance, large surface area)
Methane in mines
Increase pressure/concentration
Increases chance of a successful collision -> explosion
What does redox mean?
When oxidation and reduction take place at the same time in the same reaction (redox reactions)
What are oxidation and reduction in terms of oxygen gain or loss?
Oxidation -> oxygen added (electron lost)
Reduction -> oxygen removed (electron gained)
What is the oxidation number?
The roman numeral (oxidation number) next to transition metals
Oxidation state = oxidation number
What is redox in terms of electron transfer?
Oxidation
Lose electrons
Oxidation number increases
Reduction
Gain electrons
Oxidation number decreases
Explain how you can identify a redox equation, (using an example)?
Zn + CuSO4 → ZnSO44 + Cu
Simplify to ions present:
Zn(s) + Cu2+(aq) + SO42-(aq) →Zn2+(aq) + SO42-(aq) + Cu(s)
Remove ions that have not changed (are the same):
Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)
Split equation:
Zn(s) → Zn2+(aq) + 2e-
Cu2+(aq) + 2e- → Cu(s)
Zn lost electron to be Zn2+ (oxidize)
Cu gain electron to be Cu (reduced)
Define oxidizing agent
A substance that oxidizes another substance, becoming reduced in the process
Substance gains electrons as oxidized element loses them
Hydrogen peroxide, fluorine, chlorine
Define reducing agent
A substance that reduces another substance, becoming oxidized in the process
Loses electrons and oxidizes another substance
Carbon, hydrogen
